The art directed towards consumer microwave food packaging has experienced tremendous development in the last several years. While many factors have contributed to the rapid advancement in this art (e.g., increasing penetration of microwave ovens into households, less time available for meal preparation, etc.) perhaps the most significant has been the development of the metallized film microwave susceptor. (See, for example, U.S. Pat. No. 4,267,420 entitled Packaged Food Items and Methods for Achieving Microwave Browning. See also, U.S. Pat. Nos. 4,230,924 and 4,258,086). The metallized film susceptor in simplest form comprises a plastic film substrate and a thin deposited layer of aluminum thereon. The susceptor may additionally comprise a top layer overlaying the aluminum layer for added protection and/or for contact with the surface of a food item to be heated. Due to the thinness of the film and the reduction in structural strength of the plastic film substrate upon heating, the metallized film is conventionally mounted upon a heat resistant paperboard backing support layer.
The prior art includes numerous improvements directed towards modifying and improving one or more performance attributes of the metallized films. The prior art further includes a wide and rapidly growing variety of packaging structures which employ metallized film microwave susceptors as an integral element of the package.
With the development of this technology, microwave packaging has enjoyed a tremendous increase in popularity and usage in view of the performance, convenience, disposability and low cost of metallized film microwave susceptors.
Recently, however, developments have been made by Seaborne in formulating ceramic compositions and fabricating microwave susceptors therefrom for use in consumer microwave packaging. In U.S. Pat. No. 4,810,845 it was found that a wide variety of ceramic materials previously regarded as microwave transparent, were found to be microwave absorbing materials which could be fabricated into susceptors useful in heating devices or utensils. In part, the interest in ceramic microwave susceptors is due to the low cost of these materials which is an especially important consideration in developing disposable food packaging. Also, ceramic materials being common and inert present reduced disposal hazards. Most importantly, metallized film susceptors due to their low mass provide relatively little heat even at higher temperatures. Ceramic susceptors due to their higher mass can provide higher quantities of heat.
Surprisingly, certain specific ceramic materials previously considered to be microwave inert were discovered by Seaborne to absorb microwave energy (at 2450 Hz, i.e., microwave oven frequency) and when fabricated into susceptors could be useful in consumer food package articles for heating/browning foods. Broadly, these ceramic compositions fall into three different categories: ceramic gels, neutral ceramics, and amphoteric ceramics. Microwave ceramic gel susceptor compositions (as well as susceptors and packages comprising these susceptors) are described by Seaborne in commonly assigned U.S. Pat. No. 4,806,718 (see, equivalently, divisional U.S. Pat. No. 4,968,865). Neutral ceramics are in contra distinction to amphoteric ceramic materials. Neutral ceramics are characterized as : having no residual lattice charge. In contrast, amphoteric ceramic materials possess a residual lattice charge.
Useful selected neutral ceramic compositions for consumer microwave susceptor compositions are described by Seaborne in commonly assigned U.S. Pat. No. 4,810,845 (see equivalently, divisional U.S. Pat. No. 4,956,533). In the '845 patent the materials were ceramic materials which were characterized as having a neutral lattice charge and relatively low electrical resistivity. The preferred materials were sodium aluminum silicate clays, sodium metasilicate, kaolin and mixtures thereof. In fabricating the susceptor comprised of the ceramic susceptor material which absorbs energy and a binder, the material must be unvitrified, i.e., not subjected to conventional firing above 800.degree. F.
Useful selected amphoteric ceramic compositions are described by Seaborne in commonly assigned U.S. Pat. No. 4,818,831 (see, equivalently, divisional U.S. Pat. No. 4,965,423). In U.S. Pat. No. 4,818,831 the susceptor compositions also are comprised of a binder and certain ceramic materials in their native and amphoteric forms, as the microwave absorbing material, which ceramic materials are preferably those with residual lattice charges or an unbalance of charge in the framework or layers, such as vermiculite, bentonite, hectorite and selected micas such as Glauconite, Phlogopite, Biotite and mixtures thereof. The ceramics are activated to their amphoteric form by treatment with either acids or bases. Surprisingly, the microwave absorption activity of amphoteric ceramic microwave materials can be augmented by a therein described treatment with acids or bases.
Improvements in these ceramic microwave compositions are described too by Seaborne which additionally comprise selected metal salts as temperature profile moderators. Improved neutral ceramic compositions comprising metal salts are described in commonly assigned U.S. Pat. No. 4,825,024 (see, equivalently, divisional U.S. Pat. No. 4,950,857). Improved amphoteric ceramic compositions containing these selected metal salts are described in commonly assigned U.S. Pat. No. 4,808,780 (see, equivalently, divisional U.S. Pat. No. 4,965,427). Each of these patents are incorporated herein by reference.
Other recent developments in susceptor composition and susceptors fabricated therefrom, to prevent runaway heating, i.e., to provide heat under the influence of microwave radiation up to an upper temperature limit at which the susceptor comes to a steady state absorption of microwave energy and heating to a higher temperature, are found in commonly assigned U.S. Pat. Nos. 4,810,845 and 4,818,831. These patents deal with ceramic microwave heating susceptors.
In U.S. Pat. No. 4,959,516 there is disclosed a susceptor coating comprised of a polymeric binder with a metallic conductive particle and semiconductor particle. The metals may be aluminum, nickel, zinc or copper while the semiconductive particles are selected from carbon black, zinc oxide, titanium carbide and silicon carbide. In preparing the susceptor a dispersion of the particles in the polymeric binder is coated on a substrate and dried.
However, relatively little heat is generated upon microwave heating of such compositions. Also, the mechanical properties of such compositions are deficient. For compositions to be useful from a microwave absorption/heat output standpoint in food heating applications, the concentration of the microwave active materials must be high. However, at such high concentrations of microwave absorbing materials, the compositions lack the machinability or fabrication properties necessary for food packaging uses.
The present invention provides new and improved ceramic microwave susceptor compositions. In part, the improvements reside in new ceramic materials useful as microwave actives in ceramic susceptors. In another aspect, the improvement resides in compositions exhibiting superior mechanical properties which surprisingly nonetheless exhibit useful microwave heating and absorption performance.
The present invention is a further improvement over the articles and compositions of the parent application U.S. Ser. No. 313,930 in terms of mechanical properties of the ceramic and microwave heating performance.